In recent years, there has been increasing expectation for secondary batteries from an environmental point of view in a global scale. That is, the secondary batteries attract attention as important key items for a reduction in CO2 emissions and are used as power sources for an electric vehicle (EV), a hybrid electric vehicle (HEV), an electric motorcycle and the like, or used in combination with natural energy power generation such as solar power generation and wind power generation. Above all, lithium ion secondary batteries are particularly highly expected because energy density is high and a reduction in size and weight is expected.
When the lithium ion secondary battery is charged at a voltage not less than a set voltage, battery characteristics are seriously spoiled and deterioration in a capacity and direct current internal resistance (hereinafter referred to as DC-IR) of the battery involved in a charging and discharging cycle is accelerated. Therefore, as a general charging method for the lithium ion secondary battery, charging is performed at a constant current until a battery voltage reaches the set voltage and, thereafter, constant voltage charging for continuously controlling a charging current is performed such that the battery voltage does not rise to be not less than the set voltage. In the EV, the HEV, and the like, charging is performed with constant watt instead of the constant current. In the following description, the constant current is referred to as CC (Constant Current), the constant voltage is referred to as CV (Constant Voltage), and constant watt is referred to as CW (Constant Watt).
In the lithium ion secondary battery, in general, a carbon material is used as a negative electrode material. When the battery is charged, lithium ions emitted from a positive electrode are inserted into the carbon material of a negative electrode. During the charging, when the battery is charged with an extremely high current value exceeding the ability of the battery, an insertion reaction of the lithium ions into the carbon material is late for the charging, and the lithium ions are therefore sometimes precipitated as lithium metal. In this case, the precipitated lithium metal reacts with an electrolytic solution and changes to irreversible lithium compound not contributing to charging and discharging. As a result, the capacity of the battery decreases. The capacity of the battery decreases earlier than when the battery is used within a range of a standard charging current. That is, when charging is performed at a large charging current, time required for the charging may be reduced and, on the other hand, the battery is damaged and the life characteristics of the secondary battery are spoiled.
It is impossible to use the battery during the charging. Therefore, in general, there is a demand for a battery having a short charging time. However, it is difficult to realize both a high capacity and quick charge. In order to design a battery that may be quickly charged, the capacity has to be sacrificed to a certain degree.
Therefore, Patent Document 1 proposes to perform CC charging and CV charging in multiple stages and reduce a set current value according to a state of charge (hereinafter, SOC) to thereby reduce a charging time while reducing damage to a battery.
Like Patent Document 1, Patent Document 2 proposes to combine CC charging and CV charging in multiple stages to thereby set an electric current small at a high SOC and use a pulse wave by a ripple current to prevent a battery voltage from rising to be not less than a set voltage.
Further, Patent Document 3 proposes to, during manufacturing of a battery, form a uniform film on the surface of an active material by performing CV charging for keeping a voltage constant at a voltage at which a chemical reaction for forming a film occurs in initial charging, combine CC charging and CV charging in multiple stages to form layers of films, and improve the characteristics of the battery.
If the quick charge is realized by contrivance of only a charging method without spoiling the life characteristics of the battery, it is possible to improve convenience of the battery while maintaining the battery capacity.
The potential of the negative electrode at the time when an electric current is not fed (Open Circuit Voltage; hereinafter referred to as OCV) is present in a position higher than potential at which lithium is precipitated (0V vs. Li). In normal charging in which a current value recommended by a battery manufacturer is used, in general, potential at the time when an electric current is fed (Close Circuit Voltage; hereinafter referred to as CCV) is designed not to be the potential at which lithium is precipitated.
However, when the quick charge is performed with a larger current value, if it is assumed that the resistance of the negative electrode is constant, the CCV of the negative electrode falls to be not higher than the potential at which lithium is precipitated in a region of a high SOC where the potential of the negative electrode falls. Lithium is precipitated on the negative electrode. As a result, deterioration of the battery is accelerated by the mechanism described above.
Consequently, Patent Document 1 proposes control for reducing an electric current as the SOC increases, that is, reducing an electric current as the OCV of the negative electrode decreases. As a result, in an initial period of charging, the quick charge is possible with a large current value. However, in the latter half of the charging, since a current value decreases more than necessary, a reduction in a charging time may not be efficiently carried out.
In Patent Document 2, control for reducing an electric current through the CV charging is performed in the middle period of the charging. However, in the CV charging, it takes time to reduce the electric current to a target value. Therefore, damage to the battery progresses. Further, a current value continues to decrease even after the electric current reaches a target current value. Hence, a charging time is required more than necessary. That is, in the control of the electric current by the CV charging, a sufficient effect may not be obtained for any of the purpose of suppression of damage to the battery and the purpose of reduction in the charging time.
Further, Patent Document 3 proposes multistage charging of CC charging and CV charging same as those in Patent Documents 1 and 2. However, the multistage charging is performed only during manufacturing of a battery and has a purpose of accurately performing a chemical reaction. Therefore, the multistage charging is carried out with an extremely low current value (0.2 It) compared with the quick charge. Since acceleration of the chemical reaction is the purpose, a target of the multistage charging is a charging control method only during the initial charging. A set value of a voltage is set according to potential at which a film forming reaction is promoted rather than an SOC.
Patent Document 1: Japanese Patent Application Laid-open No. 2011-24412
Patent Document 2: Japanese Patent Application Laid-open No. H9-121462
Patent Document 3: Japanese Patent Application Laid-open No. 2002-203609